Optics in everyday application: lasers, CCD cameras,

optical fibers, liquid crystal displays, holography

Lecture 3

The gas laser

The He-Ne laser 1.

(helium-neon)

The mechanism producing population inversion

and light amplification in a He-Ne laser

plasma originates with inelastic collision of

energetic electrons with ground state helium

atoms in the gas mixture. As shown in the

accompanying energy level diagram, these

collisions excite helium atoms from the

ground state to higher energy

excited states, among them the 23S1 and

21S0 long-lived metastable states. Because

of a fortuitous near coincidence between the

energy levels of the two He metastable states,

and the 3s2 and 2s2 (levels of neon, collisions

between these helium metastable atoms and

ground state neon atoms results in a selective

and efficient transfer of excitation energy

from the helium to neon.

The He-Ne laser 2.

(helium-neon)

Gas lasers typically use a window tilted at Brewster's

angle to allow the beam to leave the laser tube.

Since the window reflects some s-polarized light but

no p-polarized light, the gain for the s polarization is

reduced, but that for the p polarization is not affected.

This causes the laser's output to be p polarized, and

allows lasing with no loss due to the window.

The physical mechanism for this can be qualitatively

understood from the manner in which electric

dipoles in the media respond to p-polarized light.

One can imagine that light incident on the surface is

absorbed, and then re-radiated by oscillating electric

dipoles at the interface between the two media. The

refracted light is emitted perpendicular to the direction

of the dipole moment; no energy can be radiated in

the direction of the dipole moment. That is, if the

oscillating dipoles are aligned along the supposed

direction of the reflection, no light is reflected at all. 1

2

n

ntg B

θr

90 Br

The He-Ne laser 3.

(helium-neon)Medical treatment

Cosmetic laser

treatment

The semiconductor laser

A laser diode is an electrically pumped semiconductor laser in which the active

medium is formed by a p-n junction of a semiconductor diode similar to that found

in a light-emitting diode. The laser diode is distinct from the optically pumped

semiconductor laser, in which, while also semiconductor based, the medium is

pumped by a light beam rather than electric current.

Light source in fiber-optic communication. High efficiency!

A semiconductor

laser application

CD (or DVD) reader & writer

(head)

The Lorentz-transformation

'' & )''(

2x

c

vttvtxx

x

c

vttvtxx

2' & )('

K → K’ : v → -v

2

2

c

v-1

1

xx’

yy’

KK’

v

The transformation describe how measurements

of space and time by two observers are related.

The Maxwell equations are invariant to Lorentz

transformation, that is a linear transformation.

The Maxwell equations are not invariant to

Galilean transformation (t=t’, x=x’+vt).

The Lorentz-transformation:

The speed of light is the same in K and K’: cc

The Doppler-effect 1.

The electromagnetic wave in K (traveling along the x axis):

The electromagnetic wave in K’ (traveling along the x’ axis):

x

λ

πft-Akx)t(EE(x,t)

22coscos 0

''

2''2cos

2vtx

λ

π-x

c

vtfAE(x,t)

c

f

1

'

1

11

2 '

1

1

2cos'

1

12

'

1

1

2cos

2

2

2

2x

c

vc

v

λπ-t

c

vc

v

fAx

c

v

c

v

λ

π-t

c

v

c

v

fAE(x,t)

c

vc

v

ff

1

1

'

c

vc

v

1

1

'

The Doppler-effect 2.

H0 = 67.0 ± 3.2 (km/s)/Mpc

Parsec: 1pc = 3,26 light-year

sHv o Hubble’s law:

All the objects observed in deep space (intergalactic

space) are found to have a Doppler shift observable

relative velocity to Earth, and to each other. The

Doppler-shift-measured velocity, of various

galaxies receding from the Earth, is proportional to

their distance from the Earth and all other interstellar

bodies. In effect, the space-time volume of the

observable universe is expanding and Hubble's law

is the direct physical observation of this process.

The motion of astronomical objects due solely to

this expansion is known as the Hubble flow.

Hubble's law is considered the first observational

basis for the expanding space paradigm and today

serves as one of the pieces of evidence most often

cited in support of the Big Bang model.

The laser ”radar”. (LDV)

The parsec is a unit of length

used in astronomy.

The Hubble-constant:

LDV is used to measure

blood flow in human tissues.

Optical activity

yx nn

Optical rotation (optical activity) is the

turning of the plane of linearly polarized light

about the direction of motion as the light

travels through certain materials. It occurs

in solutions of chiral molecules such as

sucrose (sugar), solids with rotated crystal

planes such as quartz. It is used in the sugar

industry to measure syrup concentration,

in optics to manipulate polarization, in

chemistry to characterize substances in

solution. It is being developed as a method

to measure blood sugar concentration in

diabetic people.

0

0

0

0

y

xkin

kin

y

x

E

E

e

e

E

E

oy

ox

ook

2phase

shift

Each pixel of an LCD typically consists of a layer of molecules

aligned between two transparent electrodes, and two polarizing

filters, the axes of transmission of which are (in most of the

cases) perpendicular to each other. Without the liquid crystal

between the polarizing filters, light passing through the first

filter would be blocked by the second (crossed) polarizer.

The liquid-crystal 1.

The liquid-crystal 2.

The twisted nematic effect is based on the precisely

controlled realignment of liquid crystal molecules

between different ordered molecular configurations

under the action of an applied electric field. This is

achieved with little power consumption and at low

operating voltages. temperature

The liquid-crystal 3.

The structure of LCD monitor

Wave-guide foil

Optical fibers 1.

Total internal reflection: the wave cannot pass

through and is entirely reflected. This can only

occur where the wave travels from a medium with

a higher refractive index (n1) to one with a lower

refractive index (n2). For example, it will occur

with light when passing from glass to air, but

not when passing from air to glass.

The critical angle:

2

1sinn

nc

Total internal reflection

Optical fibers 2.

laser

beam

Optical fibers 3.

Fiber with a core diameter less than about ten times the wavelength of the propagating

light cannot be modeled using geometric optics. Instead, it must be analyzed as an

electromagnetic structure, by solution of Maxwell's equations as reduced to the

electromagnetic wave equation. As an optical waveguide, the fiber supports one or

more confined transverse modes by which light can propagate along the fiber.

The waveguide analysis shows that the light energy in the fiber is not completely

confined in the core. Instead, especially in single-mode fibers, a significant fraction

of the energy in the bound mode travels in the cladding as an evanescent wave.

Symmetric waveguide

Conditions:

x: ∞ :

z: direction of traveling

Time dependance:

exp[i(ωt-βz)]

0

x

21

Maxwell eq.-s:

(j=1, 2)

zox

yox

xoyz

zjox

yjox

xjoyz

Hiy

E

HiEi

HiEiy

E

Eiy

H

EiHi

EiHiy

H

T.E.

T.M.

T.E. (5), (6) →(1) : 022

2

2

xjo

x Eky

E oook

Traveling modes: 122

22 oo kk

New variables:2

2222

21

221

o

o

k

kk

Substance 1: 1n

0212

2

xx Ek

dy

Ed

Solution(s): sin , cos )sin(

)cos(

)cos(

11

1

1

yki

kAH

ykAH

ykAE

o

z

o

y

x

Substance 2 : 0222

2

xx E

dy

Ed

y

o

z

y

o

y

y

x

ei

BH

eBH

BeE

2

2

2

2

Boundary conditions:

b

oo

b

ei

Bbki

kA

BebkA

2

2

21

1

1

sin

cos

det.=0

1

21

kbktg

2n

2121

22

2122

22

1

kk

kk

o

o

2121

211 kkbktgk o

21

2

1

obkV

bw

bku

Intr.:

222

22)(

Vwu

uVuutg

u

w

2 V 2

0k

1n

2n

Optical fibers 4.

Connectors of

multimode fibres

The structure of a typical

single-mode fiber.

1. Core: 8 µm diameter

2. Cladding: 125 µm dia.

3. Buffer: 250 µm dia.

4. Jacket: 400 µm dia.

Optical cable

Intensity distribution

Optical fibers 5.

V

V ~ a, where a is the radius of the

core. n1 and n2 are the refractive

indexes of the core and the cladding.

The electromagnetic structure can be described by solution of Maxwell's equations as

it reduced to the electromagnetic wave equation. The electromagnetic analysis may

also be require to understand behaviors such as speckle that occur when coherent

light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one

or more confined transverse modes by which light can propagate along the fiber.

Fiber supporting only one mode is called single-mode or mono-mode fiber.

The most common type of single-mode fiber has a core diameter of 8–10 m and is

designed for use in the near infrared; wavelength: = 1550 nm.

Modes of EMW in optical fibre

Op

tical fib

ers

6.

The advantages of optical fiber communication with respect to copper wire systems are:-

1. Broad Bandwidth

Broadband communication is very much possible over fiber optics which means that

audio signal, video signal, microwave signal, text and data from computers can be

modulated over light carrier wave and demodulated by optical receiver at the other end.

It is possible to transmit around 3,000,000 full-duplex voice or 90,000 TV channels over

one optical fiber.

2. Immunity to Electromagnetic Interference

Optical fiber cables carry the information over light waves which travel in the fibers

due to the properties of the fiber materials, similar to the light traveling in free space.

The light waves (EMW) are unaffected by other electromagnetic radiation nearby.

The optical fiber is electrically non-conductive, so it does not act as an antenna to

pick up electromagnetic signals which may be present nearby. So the information

traveling inside the optical fiber cables is immune to electromagnetic interference e.g.

radio transmitters, power cables adjacent to the fiber cables, or even electromagnetic

pulses generated by nuclear devices.

3. Low attenuation loss over long distances

There are various optical windows in the optical fiber cable at which the attenuation

loss is found to be comparatively low and so transmitter and receiver devices are

developed and used in these low attenuation region. Due to low attenuation of 0.2dB/km

in optical fiber cables, it is possible to achieve long distance communication efficiently

over information capacity rate of 1 Tbit/s.

4 Electrical Insulator

Optical fibers are made and drawn from silica glass which is nonconductor of electricity

and so there are no ground loops and leakage of any type of current. Optical fibers are

thus laid down along with high voltage cables on the electricity poles due to its electrical

insulator behavior.

Optical fibers 7.

The Nobel Prize in Physics 2009

Charles Kuen Kao Willard S. Boyle George E. Smith

The Nobel Prize in Physics 2009 was divided, one half awarded to Charles Kuen Kao

"for groundbreaking achievements concerning the transmission of light in fibers for

optical communication", the other half jointly to Willard S. Boyle and George E. Smith

"for the invention of an imaging semiconductor circuit - the CCD sensor".

Photography

y)I(x,thy)h(x, o

The transmission of the photoplate (or film):

iof

111

Linear range

A light-sensitive emulsion

of silver salts is applied to

a glass plate (or film).

Holography 1.

1st step: Exposition*or

*roro EEEEIII

Interference pattern (on holoplate):

Holography: lensless photography

2nd step: developement

(chemical process)

y)I(x,thy)h(x, o

The transmission of the hologram:

Ref: reference beam

(plane wave of spherical wave)

Obj: object wave

(reflected from the 3D object)

Hologram:

the recorded

interference

pattern

Holography 2.

Object wave!!!y)I(x,thy)h(x, o

Amplitude transmission:

Reconstruction:

*o

2r

2

rororror EEEE)It(IEEhhE

Off axis holography:

Real, virtual, undiffracted waves

are spatially separated

*or

*roro EEEEIII

Interference pattern (on holoplate):

3rd step: Reconstruction

Real image

Virtual image

+ =

CameraChip

type

Number of

pixels

Pixel size

[m2]

Frames per

second

Dynamic

range

max for

=633nm

Roper Sci.

MegaPlus 1.4iFT

1017*

10356.8 * 6.8 6.9 8 bit 2.7°

Roper Sci.

Megaplus 16.8iFT

4096*

40969 * 9 0.47 8 bit 2.0°

Roper Sci.

Megaplus ES 1.0IT

1008*

10189 * 9 30 8 or 10 bit 2.0°

Roper Sci.

Megaplus 4.0IT

2048*

20487.4 * 7.4 30 8 or 12 bit 2.45°

Hamamatsu

C8484-01PSI

1344*

10246.45 * 6.45 8.3 12 bit 2.81°

Duncan

DT1100PS

1392*

10404.65 * 4.65 12 8 or 12 bit 3.9°

FT: full frame: interline transfer, PSI: progressive scan interline, PS: progressive scan

The resolution of a holographic plate: 5000 lines/mm

Holography 3. Digital holography 1.

”In-line” arrangement

Digital holography 2.

3D shape measurement:

Holography 4.

Digital holography 3.Holography 5.

Gábor Dénes (Dennis Gabor): a Hungarian electrical engineer and

physicist, he invented the holography.

He received the 1971 Nobel Prize in Physics.

Digital holography 4.Holography 6.

A computer-generated hologram:

Holography 7.

X-ray, neutron, electron holography

Source: an atom

Source: atom

Reference wave: primary wave

(the original wave)

Object wave: secondary waves

scattered from the neighbor atomsComputer generated reconstruction